Post-traumatic stress disorder (PTSD) and some other anxiety disorders manifest as overgeneralization of fear memories that are derived from traumatic experiences but now applied to nonhazardous situations. The neuronal mechanisms determining the degree of memory precision versus generalization are largely unknown, and this lack of understanding impedes the development of effective therapies. Previously, I observed over- generalized contextual fear memory following functional manipulations of the medial prefrontal cortex or the hippocampus. Moreover, I discovered that nucleus reuniens of the midline thalamus which anatomically connects the prefrontal cortex to the hippocampus also plays an essential role. Selective inactivation of prefrontal neurons projecting to nucleus reuniens or direct inactivation of neurons in nucleus reuniens produced overgeneralization, while increasing neuronal activity in nucleus reuniens reduced fear generalization. These observations lead to my hypothesis that in the brain there is a tuning system bi-directionally adjusting the generalization level of fear memories, and this system is composed of the circuits between the prefrontal cortex, nucleus reuniens and the hippocampus. Low activity of the prefrontal cortex and nucleus reuniens decreases hippocampal representation of the details associated with a particular fear memory and leads to overgeneralization as seen in PTSD and anxiety; while increasing the activity level of the prefrontal cortex and nucleus reuniens will downregulate fear generalization level and provides therapeutic benefits. In the mentored phase of this proposal, I will systematically examine the roles of these three brain structures in determining fear generalization level and the functional relationship between these brain structures in encoding or retrieval of fear memories. Because my preliminary data indicate that a subset rather than the whole prefrontal neuronal population contributes to fear generalization, I will identify the particular prefrontal subpopulations of neurons which are specifically involved i the regulation of fear generalization and reveal their genetic identities with high throughput RNA sequencing techniques. Based on the above information, in the independent phase I will then perform detailed functional characterization of the specific prefrontal neuronal groups including their anatomical connections, electrophysiological properties and gene expression profiles. I will test if changes in these properties result in fear memory overgeneralization and anxiety. Successful completion of the proposed studies will offer a mechanistic account of a profound cognitive function - the determination of memory precision/generalization - and thus open up novel avenues for effective therapies to abate anxiety. Careerwise, I am interested in neuroscience research in an academic institute with a long-term goal to uncover the neuronal mechanisms underlying basic cognitive functions and to utilize the information thus obtained to treat brain disorders. I will primarily focus on but not limit my topics to various aspects of memory functions. To date, great strides have been made in neuroscience in the fields of cellular and molecular neurobiology as well as in human cognitive neuroscience; however, research bridging these two ends is lagging behind. I believe that detailed functional dissection of the neuronal circuits responsible for cognitive functions is pivotal to neuroscience now. Therefore, my immediate objective for the next 5-10 years will be to characterize in detail the connectivity and functional properties of the prefrontal cortex-thalamus pathways and to elucidate their roles in various cognitive functions such as those proposed here. I was initially trained in clinical medicine and then received scientific training in neuroscience. I have had extensive experience in both molecular and cellular neurobiology approaches and systems neuroscience techniques, which make me ideal for the proposed studies. The first phase of the current proposal will be conducted with mentoring from Dr. Thomas Sudhof and Dr. Ian Gotlib at Stanford University. Dr. Sudhof is a leading neurobiologist in studying synaptic functions and related mental disorders; Dr. Gotlib, a world renowned cognitive scientist with expertise in anxiety and depression research. With their guidance, my proposed studies shall be ensured in both scientific value and technical soundness. Stanford University has been the cradle of fundamental neuroscience findings and innovative neuroscience approaches and will furnish the best environment for the studies.